Autonomous Reconstitution of Fractured Hybrid Perovskite Single Crystals

A Self-Healing Crystal That Repairs Multiple Cracks

We report both cracking and self-healing in crystals occurring during a thermal phase transition, followed by a topochemical polymerization. A squaramide-based monomer was designed where the azide and alkyne units of adjacent molecules are positioned favorably for a topochemical click reaction. The monomer undergoes spontaneous single-crystal-to-single-crystal (SCSC) polymerization at room temperature via regiospecific 1,3-dipolar cycloaddition, yielding the corresponding triazole-linked polymer in a few days. When heated at 60 °C, the polymerization completes in a SCSC manner in 24 h. Upon continuous heating from room temperature to 110 °C, the monomer crystals develop multiple cracks, and they self-heal immediately. The cracking occurs due to a thermal phase transition, as evidenced by differential scanning calorimetry (DSC). The cracks heal either upon further heating or upon cooling of the crystals due to the topochemical polymerization or reversal of the phase transition, respectively. Increasing the heating rate leads to the formation of longer and wider cracks, which also heal instantaneously. The self-healed crystals retained their integrity and the crystal structure of the self-healed crystals was analyzed by single-crystal X-ray diffraction. The quality of the self-healed crystals and their diffraction ability conform to those of the completely reacted crystals at room temperature or at 60 °C without developing cracks. This work demonstrates a novel mechanism for self-healing of molecular crystals that could expand the horizon of these materials for a plethora of applications.

Maneuverability and Processability of Molecular Crystals

Crystal adaptronics, a burgeoning field at the intersection of materials science and engineering, focuses on harnessing the unique properties of organic molecular crystals to achieve unprecedented levels of maneuverability and processability in various applications. Increasingly, ordered stacks of crystalline materials are being endowed with fascinating mechanical compliance changes in response to external environments. Understanding how these crystals can be manipulated and tailored for specific functions has become paramount in the pursuit of advanced materials with customizable properties. Simultaneously, the processability of organic molecular crystals plays a pivotal role in shaping their utility in real-world applications. From growth methodologies to fabrication techniques, the ability to precisely machine these crystals opens new avenues for engineering materials with enhanced functionality. These processing methods enhance the versatility of organic crystals, allowing their integration into various devices and technologies, and further expanding the potential applications. This review aims to provide a concise overview of the current landscape in the study of dynamic organic molecular crystals, with an emphasis on the interconnected themes of operability and processability.

Ferroelastic ionic organic crystals that self-heal to 95

The realm of self-healing materials integrates chemical and physical mechanisms that prevent wear and fracturing and extend the operational lifetime. Unlike the favorable rheology of amorphous soft materials that facilitates efficient contact between fragments, the efficiency of recovery of atomistically ordered materials is restricted by slower interfacial mass transport and the need for ideal physical alignment, which limits their real-world application. We report drastic enhancements in efficiency and recovery time in the self-healing of anilinium bromide, challenging these limitations. Crystals of this material recovered up to 49% within seconds and up to 95% after 100 min via ferroelastic detwinning. The spatial evolution of strain during cracking and healing was measured in real time using digital image correlation. Favorable alignment and strong ionic bonding across the interface of partially fractured crystals facilitate self-healing. This study elevates organic crystals close to the best-in-class self-healing polymers and sets an approach for durable crystal-based optoelectronics.

Unraveling the Degradation Mechanisms of Perovskite Solar Cells under Mechanical Tensile Loads

The short longevity of perovskite solar cells (PSCs) is the major hurdle toward their commercialization. In recent years, mechanical stability has emerged as a pivotal aspect in enhancing the overall durability of PSCs, prompting a myriad of strategies devoted to this issue. However, the mechanical degradation mechanisms of PSCs remain largely unexplored, with corresponding studies mainly limited to perovskite single crystals, neglecting the complexity and nuances present in PSC devices based on polycrystalline perovskite thin films. Herein, we reveal the underlying mechanisms of the mechanical degradation of formamidinium-based PSCs, which are the most prevalent high-performance PSC candidates. Under uniaxial tensile loads, we found that the degradation is mainly attributed to the sequential increase in the density of micropores and halide defects within the perovskite films. This phenomenon is consistent across various perovskite compositions and environmental conditions. Our findings elucidate mechanistic insights for more targeted mitigation strategies aimed at addressing the mechanical degradation of PSC devices.

Highly efficient in crystallo energy transduction of light to work

Various mechanical effects have been reported with molecular materials, yet organic crystals capable of multiple dynamic effects are rare, and at present, their performance is worse than some of the common actuators. Here, we report a confluence of different mechanical effects across three polymorphs of an organic crystal that can efficiently convert light into work. Upon photodimerization, acicular crystals of polymorph I display output work densities of about 0.06-3.94 kJ m-3, comparable to ceramic piezoelectric actuators. Prismatic crystals of the same form exhibit very high work densities of about 1.5-28.5 kJ m-3, values that are comparable to thermal actuators. Moreover, while crystals of polymorph II roll under the same conditions, crystals of polymorph III are not photochemically reactive; however, they are mechanically flexible. The results demonstrate that multiple and possibly combined mechanical effects can be anticipated even for a simple organic crystal.

Flexible molecular crystals for optoelectronic applications

The cornerstones of the advancement of flexible optoelectronics are the design, preparation, and utilization of novel materials with favorable mechanical and advanced optoelectronic properties. Molecular crystalline materials have emerged as a class of underexplored yet promising materials due to the reduced grain boundaries and defects anticipated to provide enhanced photoelectric characteristics. An inherent drawback that has precluded wider implementation of molecular crystals thus far, however, has been their brittleness, which renders them incapable of ensuring mechanical compliance required for even simple elastic or plastic deformation of the device. It is perplexing that despite a plethora of reports that have in the meantime become available underpinning the flexibility of molecular crystals, the "discovery" of elastically or plastically deformable crystals remains limited to cases of serendipitous and laborious trial-and-error approaches, a situation that calls for a systematic and thorough assessment of these properties and their correlation with the structure. This review provides a comprehensive and concise overview of the current understanding of the origins of crystal flexibility, the working mechanisms of deformations such as plastic and elastic bending behaviors, and insights into the examples of flexible molecular crystals, specifically concerning photoelectronic changes that occur in deformed crystals. We hope this summary will provide a reference for future experimental and computational efforts with flexible molecular crystals aimed towards improving their mechanical behavior and optoelectronic properties, ultimately intending to advance the flexible optoelectronic technology.

Autonomous self-healing organic crystals for nonlinear optics

Non-centrosymmetric molecular crystals have a plethora of applications, such as piezoelectric transducers, energy storage and nonlinear optical materials owing to their unique structural order which is absent in other synthetic materials. As most crystals are brittle, their efficiency declines upon prolonged usage due to fatigue or catastrophic failure, limiting their utilities. Some natural substances, like bone, enamel, leaf and skin, function efficiently, last a life-time, thanks to their inherent self-healing nature. Therefore, incorporating self-healing ability in crystalline materials will greatly broaden their scope. Here, we report single crystals of a dibenzoate derivative, capable of self-healing within milliseconds via autonomous actuation. Systematic quantitative experiments reveal the limit of mechanical forces that the self-healing crystals can withstand. As a proof-of-concept, we also demonstrate that our self-healed crystals can retain their second harmonic generation (SHG) with high efficiency. Kinematic analysis of the actuation in our system also revealed its impressive performance parameters, and shows actuation response times in the millisecond range.

A-Site Cation Dependence of Self-Healing in Polycrystalline APbI3 Perovskite Films

In terms of sustainable use, halide perovskite (HaP) semiconductors have a strong advantage over most other classes of materials for (opto)electronics, as they can self-heal (SH) from photodamage. While there is considerable literature on SH in devices, where it may not be clear exactly where damage and SH occur, there is much less on the HaP material itself. Here we perform "fluorescence recovery after photobleaching" (FRAP) measurements to study SH on polycrystalline thin films for which encapsulation is critical to achieving complete and fast self-healing. We compare SH in three photoactive APbI₃ perovskite films by varying the A-site cation ranging from (relatively) small inorganic Cs through medium-sized MA to large FA (the last two are organic cations). While the A cation is often considered electronically relatively inactive, it significantly affects both SH kinetics and the threshold for photodamage. The SH kinetics are markedly faster for γ-CsPbI₃ and α-FAPbI₃ than for MAPbI₃. Furthermore, γ-CsPbI₃ exhibits an intricate interplay between photoinduced darkening and brightening. We suggest possible explanations for the observed differences in SH behavior. This study's results are essential for identifying absorber materials that can regain intrinsic, insolation-induced photodamage-linked efficiency loss during its rest cycles, thus enabling applications such as autonomously sustainable electronics.

Topotactic, Vapor-Phase, In Situ Monitored Formation of Ultrathin, Phase-Pure 2D-on-3D Halide Perovskite Surfaces

Two-dimensional (2D) halide perovskites, HaPs, can provide chemical stability to three-dimensional (3D) HaP surfaces, protecting them from exposure to ambient species and from reacting with contacting layers. Both actions occur with 2D HaPs, with the general stoichiometry R₂PbI₄ (R: long or bulky organic amine) covering the 3D ones. Adding such covering films can also boost power conversion efficiencies of photovoltaic cells by passivating surface/interface trap states. For maximum benefit, we need conformal ultrathin and phase-pure (n = 1) 2D layers to enable efficient tunneling of photogenerated charge carriers through the 2D film barrier. Conformal coverage of ultrathin (<10 nm) R₂PbI₄ layers on 3D perovskites is challenging with spin coating; even more so is its upscaling for larger-area devices. We report on vapor-phase cation exchange of the 3D surface with the R₂PbI₄ molecules and real-time in situ growth monitoring by photoluminescence (PL) to determine limits for forming ultrathin 2D layers. We characterize the 2D growth stages, following the changing PL intensity-time profiles, by combining structural, optical, morphological, and compositional characterizations. Moreover, from quantitative X-ray photoelectron spectroscopy (XPS) analysis on 2D/3D bilayer films, we estimate the smallest width of a 2D cover that we can grow to be <5 nm, roughly the limit for efficient tunneling through a (semi)conjugated organic barrier. We also find that, besides protecting the 3D against ambient humidity-induced degradation, the ultrathin 2D-on-3D film also aids self-repair following photodamage.

Self-Healing Ability of Perovskites Observed via Photoluminescence Response on Nanoscale Local Forces and Mechanical Damage

The photoluminescence (PL) of metal halide perovskites can recover after light or current-induced degradation. This self-healing ability is tested by acting mechanically on MAPbI3 polycrystalline microcrystals by an atomic force microscope tip (applying force, scratching, and cutting) while monitoring the PL. Although strain and crystal damage induce strong PL quenching, the initial balance between radiative and nonradiative processes in the microcrystals is restored within a few minutes. The stepwise quenching-recovery cycles induced by the mechanical action is interpreted as a modulation of the PL blinking behavior. This study proposes that the dynamic equilibrium between active and inactive states of the metastable nonradiative recombination centers causing blinking is perturbed by strain. Reversible stochastic transformation of several nonradiative centers per microcrystal under application/release of the local stress can lead to the observed PL quenching and recovery. Fitting the experimental PL trajectories by a phenomenological model based on viscoelasticity provides a characteristic time of strain relaxation in MAPbI3 on the order of 10-100 s. The key role of metastable defect states in nonradiative losses and in the self-healing properties of perovskites is suggested.